Images are transcribed in digital form with as low a definition as possible so as to limit the number of bits used. When an image is transcribed in digital form, it is divided up into as loose a grid as possible of regularly spaced rectangular zones to which digital values are ascribed representing their average tone. When the image is restored, a set of rectangular zones divided up in the same manner is reproduced, with average tones being ascribed to said zones which correspond to said digital values by choosing between various arbitrary graphic configurations which give these same average tones.
The features of an image which coincide with rapid changes in tone do not follow the grid layout used for digital transcription but cut across the zones thereby imparting irregular tones thereto. Since such irregular tones are not taken into account during digital transcription, said features are attenuated on reproduction, thus producing a blurred effect which is one of the main obstacles to reducing definition in a digital image.
The present invention can be used to reduce this defect, in particular in the case where the original image is an image of a face which is transcribed into digital code by means of continuous horizontal line scanning and in which the blurred effect is most particularly noticeable on the vertical features of the profile of the nose against a low contrast background.